Diabetes is a disorder characterized by metabolic defects in production and utilization of glucose resulting in a failure to maintain appropriate blood sugar levels in the body. Diabetes in humans can be defined as a disorder corresponding to a fasting plasma glucose concentration greater than 125 mg/dL, or a plasma glucose concentration greater than 199 mg/dL two hours after ingestion of a 75 g oral glucose load. Two major forms of diabetes are type 1 diabetes (T1D) and type 2 diabetes (T2D). T1D is an autoimmune disorder resulting in destruction of beta-pancreatic cells and an absolute deficiency of insulin, the hormone that regulates glucose utilization. By contrast, T2D can occur with normal or even elevated levels of insulin from the inability of tissues to respond appropriately to insulin. Most T2D patients have impaired insulin sensitivity. Insulin secretion cannot compensate for the resistance of peripheral tissues to respond to insulin. Many T2D patients are obese. Type 1.5 diabetes (late autoimmune onset in adults) shows some characteristics of T1D and T2D.
Zinc is required for insulin biosynthesis and processing. Two zinc ions are complexed in a hexameric form of proinsulin, which is ultimately processed to insulin.
Gene SLC30A8 encodes an islet zinc transporter, ZnT8, predominantly expressed in the alpha and beta-pancreatic cells. It is expressed ten-fold higher in these cells than any other zinc transporters. Its expression is localized in secretory granule membranes where it transports zinc ions from the cytosol into vesicles. The protein has six transmembrane segments with both N and C-termini being intracellular. The fourth and fifth transmembrane segments, which are connected intracellularly by a histidine rich loop, form the ion channel for passage of zinc.
Genome-wide association studies have found single-nucleotide polymorphisms in the SLC30A8 gene with significant differences in genotype frequencies between cases of type 2 diabetes and controls (Sladek, R. et al., 2007, Nature, 445:881-885). Loci in the SLC30A8 gene were found that contain variants that confer T2D risk.
The major allele rs13266634 (C), encoding an arginine at position 325 (R325; W325R) was identified as a risk allele for T2D. (Sladek, 2007, supra; Rutter G. A. and Chimienti, F. 2104, Diabetologia 58:31-36). Sladek et al. (2007) supra report that the major allele at rs13266634 (C) is the ancestral allele and suggest that the ancestral allele was adapted to the environment of ancient human populations but today in a different environment, it increases disease risk. The minor allele rs13266634 (T) encodes an arginine to tryptophan substitution at position 325 (R325W; W325) in the SLC30A8 protein, encoding a protein with increased zinc transport activity. Individuals carrying the T allele (W325) have a reduced risk of T2D.
Various mutations in SLC30A8 have been reported. A common mutation R325W due to an rs13266634 C>T polymorphism, located in the C-terminal cytoplasmic tail, has been associated with increased levels of insulin and glucose and susceptibility to T2D (14% increased risk of T2D per R325 allele). Individuals with this mutation have reduced first phase insulin release and reduced proinsulin conversion. This mutation has been reported to reduce zinc transport activity but not change SLC30A8 expression significantly. Several rare mutations in which stop codons, frameshifts, splice site or initiator codon variants cause protein truncation, and presumably substantial loss of function of the truncated protein expressed, have a protective effect against T2D in heterozygous form in humans (Flannick et al., Nat. Genet. 46(4):357-363 (2014)). On average these mutations reduce the risk of T2D by 60%. Mice with a homozygous knockout of SLC30A8 have a heterogeneous phenotype depending on gender, genetic background, and nature of the knockout (e.g., all tissues or alpha or beta-pancreatic cell specific) varying from no overt effects to symptoms of T2D (Silva Xavier et al., J. Diabetes Res., doi: 10.1155/2013/906590, Epub Apr. 11. 2013). A beta cell specific knockout has been reported to have increased proinsulin level, defects in insulin crystallization, and delayed and reduced first phase glucose-stimulated insulin secretion (Wijesekara et al., Diabetologia 53:1656-1668 (2010)) or decreased pancreatic zinc levels, increased insulin release and increased clearance of insulin by the liver (Tamaki et al., J. Clin. Invest. 123: 4513-4524 (2013)). However, the phenotypes of these SLC30A8 knockout mice have been inconsistent, making them unsuitable for a T2D disease model.